This study proposes NMR structural interpretation of the recognition between the HIV protein Nef and the SH3 domain in Src family kinase Hck, particularly the role of two distant proline-rich motifs, one long and one short. It is proposed to: 1) express and purify (15)N labeled long proline-rich motif and Hck SH3 domain; 2) determine the structure of SH3 and its complexes with the long motif; 3) prepare an expression vector for a segment of Nef protein by bilaterally truncating the gene region shortly beyond the motifs, using standard recombinant DNA and plasmid insertion techniques, with variant and mutant amino acid Nef sequences (from NL4-3) in the motifs; 4) express and purify these (13)C and (15)N labeled proteins; 5) examine the complete solution structure of at least one of these complexed with SH3; and 6) explore interaction dynamically between distant motifs via (15)N and (13)C relaxations. In view of 1) the generally conserved quality of the motifs, 2) their essentiality in modulation of Nef communicating to cellular signaling paths (e.g., SH3 of Hck), 3) the existence of variant amino acid substitutions in the motifs in patient-derived Nef, and 4) the repressive effect of the short motif on the Hck binding, when altered, it is felt that these studies may be pivotal in elucidating the still-unclear role of Nef in HIV, and in providing clues to therapeutic and pharmacological interventions in the course of infection.